The present invention relates to a development device provided in image forming apparatuses such as electrophotographic copiers, printers, facsimile machines, etc. and a development method, and specifically to a color image forming apparatus comprised of a plurality of development devices.
As development methods of an electrophotographic apparatus, have been known a normal development method employed in common electrophotographic copiers, etc. and a reversal development method employed in digital printers, digital electrophotographic copiers, etc. The reversal development method is a method in that generally, laser, LED, etc. are employed as a light source, and a latent image formed on an image bearing body (hereinafter referred to as a photoreceptor) by charging and exposure is developed employing a toner charged with the same polarity as that of the charge on the photoreceptor to obtain a toner image. For example, when a photoreceptor is negatively charged, the toner is negatively charged, and development is carried out utilizing the difference in potential generated by exposure, and a toner image is formed on the photoreceptor. After development, a transfer material is charged in opposite as of the toner employing a transfer device using corona discharging, etc. and the toner image on the photoreceptor is transferred onto the transfer material. Thereafter, the potential of the transfer material is lowered by AC corona discharging or DC corona discharging; adhesion between the transfer material and the photoreceptor is lowered; the transfer material is then separated from the surface of the photoreceptor, and the subsequent process follows.
In the conventional development device, a rotating developer material bearing device (development sleeve) is arranged adjacent to a rotating photoreceptor. The development sleeve is formed in a hollow cylindrical shape, and is housed in a housing with an opening in the side facing the above-mentioned photoreceptor. In the development sleeve, development bias voltage with which AC voltage of 2,700 volts and 8,000 Hz are superposed is applied to DC voltage of -650 volts. The developer material bearing body comprises a fixed magnet in the interior and the circumferential surface bears a two-component developer material prepared by mixing toner particles with magnetic granules (carrier).
The development device is composed of a development unit housing loading a two-component developer material comprised of the above-mentioned toner and carrier, a development sleeve as a developer material transport means with a magnetic field generating means (magnet roll) comprising a fixed magnet in the interior, a developer material supply roller (hereinafter, referred to as supply roller), a developer material layer regulating member, which regulates the thickness of the developer material layer on the above-mentioned development sleeve, and a developer material stirring screw (stirring screw). In the toner supply opening section which is opened in the upper position of the above-mentioned development unit housing, the toner supplied from a toner cartridge to the interior of the above-mentioned development unit housing is stirred and mixed with the developer material loaded into the above-mentioned development unit housing to prepare the toner uniform in concentration and the resulting toner is supplied onto the circumferential surface of the above-mentioned development sleeve.
Of the developer materials on the development sleeve, the toner is only employed for development in the development range of an image bearing body (photoreceptor), and depending on an image pattern, the hysteresis of an image remains on the development sleeve. Irrespective of developing an image pattern having the same image and density, the image defect occasionally causes a density difference termed "ghosting". The conventional technique to minimize this ghosting is a rubber material termed a scraper, which is brought into contact with the development sleeve and the image hysteresis is eliminated by scraping the developer material from the development sleeve after development to minimize the ghosting problem.
However, in the conventional scraper method in which the scraper is in contact with the development sleeve, several problems are caused such that the developer material is not perfectly removed by the contact of the development sleeve with the scraper; the results are a deterioration of the developer material when it is scraped, and scratches and abrasion on the development sleeve.
Furthermore, by adjoining the magnetic field of the development sleeve to a portion termed "detaching magnetic pole" having the same magnetic polarity as that of the development sleeve, the developer material can be detached to some extent from the sleeve in the repulsive magnetic field of the detaching magnetic pole. However, only by providing the detaching magnetic pole, the developer material cannot be perfectly detached from the development sleeve. By optimizing the detaching magnetic pole and peripheral processes, the ghosting is minimized fairly well. However, the result is unsatisfactory.